Ultrasonic intrusion detection systems employing turbulence discrimination

ABSTRACT

There is disclosed an ultrasonic intrusion detection system which is selectively activated and deactivated according to the existence of air turbulence effects as provided by a heating and/or cooling system. The ultrasonic system is deactivated when a heating system is operated and activated when a predesired temperature is achieved resulting in the heating system being inoperative. The operation of the system therefore prevents the ultrasonic intrusion system from operating during air turbulence.

United States Patent Cohen [76] Inventor: Leopold Cohen, 518 Seven OaksRd., Orange, NJ. 07051 [22] Filed: Oct. 17, 1973 [21] Appl. No.: 407,062

[52] 1.1.8. Cl. 340/258 A, 340/1 R, 340/276 [51] Int. Cl. G08b 13/16[58] Field of Search 340/258 A, 258 B [56] References Cited UNITEDSTATES PATENTS 2,779,935 1/1957 Loudon et a1. 340/258 A 2,782,405 2/1957Weisz et a1. 340/258 A 2,972,133 2/1961 MacDonald 340/258 A 3,111,65711/1963 Bagno 340/258 A 3,573,817 4/1971 Akers 340/261 X [451 Feb. 11,1975 3,629,812 12/1971 Amato 340/258 A 3,638,210 1/1972 Hankins et a1.340/258 A 3,725,888 4/1973 Solomon... 340/258 A 3,727,216 4/1973 Antonio340/258 A Primary Examiner-David L. Trafton Attorney, Agent, orFirm-Arthur L. Plevy [57] ABSTRACT 10 Claims, 3 Drawing Figures 551 BQEE EEH/L'HM e I n M? :2 t l FORCED AIR A5 I I HEA'HNG THERMosTATUHEQSONIC l 1 AND/0R CONTROL I I AIR cono- I 20 I ITIONING I I CONTROL 1l SYSTEM INTRUDER I I DETECTOR I t 26 i INTRUSION I I BACKUP I I 'svsrsmI l 30 l L. .J

CENTRAL OFFICE TNTTHTED 1 I975 3.866.198

- SHEET 2 OF 2 HEATING SYSTEM 80 f To Q 8| ULTRASONIC '1 82 SOURCETHERMO- i STAT I. O"--- FROM ULTRASONIC SOURCE I87 1 DET. DOPPLER T I20DET.

TO COMMUNlCATlQN LINE ULTRASONIC INTRUSION DETECTION SYSTEMS EMPLOYINGTURBULENCE DISCRIMINATION BACKGROUND OF INVENTION This invention relatesto intruder detection systems employing ultrasonics and moreparticularly to such a system incorporating structure to preventspurious or false alarms.

The prior art is replete with a number of systems commonly designated asintrusion detector system and employed on premises to indicate theundesired presence of an intruder as a burgular. As such, the systems ingeneral use employ many different techniques for detection of such anintruder and for notifying the proper authorities or for otherwiseactivating an alarm.

Thus, the prior art discusses microwave systems which employ radar-likesignal frequencies, light-beam systems, and ultrasonic systems.

When one is concerned with ultrasonics, one is operating with relativelylow frequencies and in the upper audio range. Conventionally, ultrasonicsignals are not audible to the human ear, but are easily andeconomically generated at relatively large power levels. Ultrasonicdetection is an extremely effective system in monitoring enclosed areasand has great advantage, in that they are relatively reliable andconsistent.

Generally, the term ultrasonics (or Supersonics) refers to soundvibrations, say for example above about 18,000 Hz. In any event,researchers have attained ultrasonic frequency of the order of millionHz or cycles per second. Ultrasonic frequencies can be provided by anumber of conventional techniques such as the Galton pipe,magnetostriction generators or piezoelectric devices and are used inmany technical applications, which, of course, include intrusiondetection schemes. In such schemes an ultrasonic transmitter produces aultrasonic frequency which is utilized to saturate an area bypropagating ultrasonic waves. The intrusion systems employ the Dopplereffect to detect the presence of an intruder by monitoring movement.

Basically, the Doppler effect is provided when a vibrating source ofwaves (such as an ultrasonic transmitter) is impinging on a movingtarget (such as an intruder). Generally as the source approaches thetarget, the frequency observed at a receiving location is higher thanthe frequency emitted by the source. If the source is receding, theobserved frequency is lower. It is, of course understood that motion isrelative and hence, the source is stationary and the observer or targetmoves with respect to the same.

The sensitivity of an ultrasonic Doppler system employing Dopplerdetection is extremely great as indicated, as well as the fact, that thesystem will not penetrate walls or normal barriers as the ultrasonicwaves bounce off or impinge from such surfaces. Hence, one canaccurately monitor a closed or walled location, such as a room, store,warehouse and so on, without fear that the waves will penetrate beyondthe area as is the case in certain microwave systems.

However, it is well known that ultrasonic frequencies are randomlyproduced by all types and sorts of vibrating equipment as well as byharmonics of whistles and such vibrations. Thus, the prior art, fullycognizant of the utility of such systems, also understood thatdiscrimination had to be made of such spurious sources to avoid falsealarms.

Literally, there are numerous factors which can give rise to falsesignals. Some of these factors are relatively easy to overcome as theyare related to the magnitude of the Doppler signal received and can beadjusted for by the use of limiting circuits and so on.

In any event, there are certain system noises which exist which areextremely difficult to discriminate against. These spurious signals aregenerated both within the ultrasonic range and which also produceDoppler frequency signals of a magnitude sufficient to trigger theultrasonic alarm. Examples of interfering sources encompass vibratingwater pipes, horns, shattering glass and so on, all of which can produceundesirous false alarms.

The prior art has implemented devices and structure to avoid theseproblems. Most solutions utilize bandwidth limiting or frequencyselection and so on to afford discrimination. Examples of typicalapparatus are evidenced by US. Pat. No. 3,513,463 entitled SOUND MONITORINTRUDER ALARM SYSTEM issued on May 19, I970 to L. A. Stevenson, Jr., etal, US. Pat. No. 3,304,457 entitled ALARM SYSTEM issued on Feb. 14, 1967to B. Bristol III. There are of course, many other patents too numerousto mention.

In any event, a major problem and threat to the ultrasonic intrusionsystem resides in the action of air turbulence. Thus, normal airturbulence as that produced by the operation of a heating fan, airconditioning unit and so on, produces air currents or frequencies whichare within the range of the Doppler shift or frequency caused by anintruder. Such turb ulences as generated create false signals and areextremely difficult to discriminate against. One cannot decrease thesensitivity of the system to avoid these spurious signals as the systemwould become inoperative. Thus, the prior art proposed electronicsolutions which systems alter time constants, employ narrow bandamplifiers or ignore short interval spurious signals. Examples ofsystems which are concerned with the problem and propose solutions areevidenced by at least the following patents:

1. US. Pat. No. 2,794,974 entitled COMPENSA- TION FOR TURBULENCE ANDOTHER EF- FECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et alissued on June 4, 1957.

2. U.S. Pat. No. 3,111,657 entitled COMPENSA- TION FOR TURBULENCE ANDOTHER EF- FECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et alissued on Nov. l9, 1963, and

3. US. Pat. No. 3,638,2l0 entitled INTRUSION ALARM WITH TURBULENCECOMPENSA- TION by T. C. Hankins, et al issued on Jan. 25, 1972.

In spite of the proposed solutions, the problem still exists to theextent that many suppliers of ultrasonic intrusion systems, will notinstall the system in an area where a forced air heating system or anair conditioning system is utilized.

It is therefore an object of the present invention to provide anintrusion detection system which offers improved discrimination againstair turbulence effects and which system permits operation of ultrasonicsunder conditions and in locations not previously available.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT An intrusion detection systemfor monitoring a predetermined area when activated, is of the typeemploying a selected frequency transmitter and a receiver responsive tosaid transmitted frequency as determined by a Doppler Shift due to thepresence of an intruder in said area, the combination with said systemincluding apparatus for operating said system in said area in thepresence of a generating source of air current turbulence, which sourceis selectively operated according to environmental conditions of saidarea, and which air turbulence undesirably caused said system to falselyindicate the presence of said intruder, comprising first meansresponsive to the operation of said selectively operated generatingsource to provide a control signal indicative of said operation andsecond means coupled to said intrusion system and responsive to saidcontrol signal for deactivating said system during the presence of saidcontrol signal, whereby said intrusion system is monitoring said areaonly when air turbulence indicative of the operation of said generatingsource is not present.

BRIEF DESCRIPTION OF FIGURES FIG. 1 is a block diagram of an intrusiondetection system according to this invention.

FIG. 2 is a block diagram partly in schematic form of the intrusionsystem depicted in FIG. 1.

FIG. 3 is a schematic diagram of an alternate operating embodiment of acontrol system according to this invention.

DETAILED DESCRIPTION OF FIGURES Referring to FIG. 1, there is shown anarea which is to be protected by an intrusion system according to thisinvention. Area 10 may be a store area, a room or any other typicalwalled establishment. The area as ew" i a ly nq ud a qr q eirhsafinsansb/or air conditioning system 12. Such systems are well known and are usedfor heating or cooling the area 10 according to the temperature of theenvironment as monitored by a thermostat control 14. Before describingthe operation of the system of FIG. 1 in greater detail an explanationof the difficulties to be experienced by an ultrasonic detection systemdue to the presence of the heating system or generating source of aircurrents 12 will be given.

Such systems 12 operate in conjunction with a fan or blower to circulateand move warmed or cooled air throughout the area '10.

Thus, when the heating system 12 is operating, the fan associatedtherewith causes the air within the area 10 to accelerate or move ratherrapidly and sporadically. The emitted ultrasonic waves from thetransmitter also circulate or propagate throughout the area 10. Theradiations or ultrasonic waves must propagate through the turbulent airand if the air is moving, the radiations are affected exactly as if theyhad been reflected from a moving object such as an intruder. Therefore achange in temperature of the area 10 will cause the velocity to vary,and such a change in velocity or frequency appears as a Doppler shift.

Normal temperature changes are ery low and present no particularproblem, but the operation of the fan incorporated in the heating system12 causes rapid changes which appear as valid Doppler shifts.

The prior art decreased the sensitivity of the detection system tocircumvent such effects resulting in a substantial decrease of systemefficiency. Others, as described in US. Pat. No. 2,794,974 employed thefrequency range associated with turbulence and discriminated against thesame. This approach is also unacceptable as the heating systems 12 haveprogressed in operation, use more powerful fans and motors and inessence, provide spurious Doppler by products over the major portion ofthe ultrasonic detection systems operating spectrum. Thus, in actualpractice, one cannot conveniently without a loss in efficiency, operatean ultrasonic system in the presence of such turbulence.

Accordingly, FIG. 1 depicts an ultrasonic intrusion system 15. Thesystem 15 is typical and is well known and generally comprises anultrasonic transducer or transmitter assembly 16 which provides afrequency in the ultrasonic range (typically from 19 to 30 KHZ). Thetransmitter 16 may be a piezoelectric microphone and so on and isenergized by typical transistor or vacuum tube amplifying andoscillating circuitry.

The transmitter 16 conventionally saturates the area 10 with ultrasonicpropogations. A receiving antenna or transponder 17 is also included inthe system 15. The receiver unit 17 may be the same transponder as thetransmitter 16 and the same are typically time-shared. In any event, thereceiver is responsive to reflected ultrasonic transmissions and if anintruder is present, the detecting circuitry associated with thereceiver is responsive to the well known Doppler shift to provide analarm signal representative of the presence of an intruder. This signalis evidenced by module 20 designated as Intruder Detect. The presence ofan alarm or a valid Doppler shift may be converted to an AC signal fortransmission over a conventional communications link as a telephonecircuit. For example, the operation of the intrusion system may bemonitored at a remote location, such as a police station or anindependentsecurity agency. Hence, if an intruder is present and a validDoppler shift is detected via module 20, a signal is sent to the remotelocation to notify the authorities. If a telephone line is used thissignal may include modulation on a typical telephone carrier frequencyas a 2,500I-Iz signal, or the signal may be a predetermined signal, saya unmodulated signal within the telephone line bandwidth (300 Hz to3,000 Hz). Alternatively, it is understood that the output of intruderdetect 20 may be coupled directly to an alarm as a bell and so on tocreate an alarm condition at the area 10.

Typically a telephone is used for remote monitoring and as such a normalmode signal is sent when there is no intrusion, to ascertain that thearea 10 is in fact being monitored and to determine that thesystem isoperating. By way of example only, assume that module 20 provides asteady 1,000Hz signal via the telephone unit, which may be a modem andthence via the telephone channel or lines 24 to a central office orremote location. During a valid Doppler shift, the detection unit 20 mayconvert this to 2,000 Hz or may provide a modulation on the l,000 I-Izcarrier. This condition is detected at the remote location and serves tonotify the authorities. Such techniques are typical and well known andthe prior art is replete with many examples of typical systems.

Also included in FIG. 1 is a control circuit 26 which has an inputcoupled to the thermostat 14 and an output coupled to the ultrasonicintrusion system 15. Another output of the control circuit 26 is coupledto a separate input of the Intruder Detection Module 20. There is alsoincluded a system designated as a Intrusion Back-up System 30. TheIntrusion Back-up system 30 has an input coupled to control circuit 26and an output coupled to the intrusion detector 20. Specifically, thedetails of operation of the apparatus of FIG. 1 can be best described asa dual-mode operation and will be discussed as such.

MODE 1 HEATING SYSTEM IS ON AND UL- TRASONIC SYSTEM IS OFF It has beendetermined that reliability cannot be achieved in the presence ofturbulence as generated by the heating and cooling system operation.Thus by monitoring the thermostat 14, one can easily ascertain that theheating system 12 is operating. The control circuit which may be aswitch activated by the thermostat serves to deactivate the ultrasonicsystem. Deactivation of this system can be accomplished in a number ofways, as by removing bias from the transmitter unit 16 and or thereceiver, blocking a signal and so on. During this condition there canbe no interference due to turbulence as the ultrasonic system ceasesoperation. The control circuit 26 simultaneously serves to generate thenormal signal via the intruder detect module 20, so that the centraloffice does not receive an intruder alarm. In the above example, thiscontrol circuit would therefore provide a 111MHz signal when theultrasonic system 15 is deactivated.

The control circuit 26, during this condition may then simultaneouslyactivate the Intrusion Back-up system 30. System 30 is also aconventional type such as a light beam security system and operates onlywhen the ultrasonic system 15 is deactivated.

The system 30 is immune to turbulence, but may possess other undesirablefeatures as compared to the overall more reliable ultrasonic system.Hence, while the system 15 is off the system 30 provides back-up supportand if a light beam is interrupted or otherwise, the back-up system 30provides a alarm signal via module 20 notifying the authorities of theintrusion, as above indicated.

The philosophy of operation and utility of the abovedescribed system isas follows:

1. There can never be turbulence interference as the ultrasonic system15 is off when the heating system IS on.

2. It is recognized that a heating or cooling system, in the majority ofapplications does not operate continuously. This is especially true intemperate climates and further valid when one considers that the majorconcern of intrusion occurs in the late hours or in the evening when thetemperature is more or less stabilized. Thus, the heating or coolingsystem is on for relatively short intervals thereby assuring that theultrasonic system is monitoring the area 10, at highest efficiencyduring all times the heating system is off. In any event, it isrecognized that certain conditions of weather might cause the heatingsystem to operate continuously, thus eliminating ultrasonic protectionfor extended periods. This condition is rare in temperate climates, asis easily ascertained.

3. The system still provides intrusion detection when the ultrasonicsystem is off via system 30, which because of the system type is immuneto spurious conditions as would effect the ultrasonic system. MODE IITHE HEATING SYSTEM IS OFF AND THE ULTRASONIC SYSTEM IS ON As indicatedabove when the temperature of the environment of area is within thedesired range the heating system 10 is not operating as determined bytypical thermostat 14 operation. Thus the control circuit 25 activatesthe ultrasonic system 15 and disables or deactivates system 30, thisaction results in a normal ultrasonic operative mode and the area 10 isso secured.

FIG. 2 depicts certain aspects of FIG. 1 in greater detail.

A typical thermostat 40 is shown and numerous examples are availablecommercially. Generally thermostat 40 responds to temperature and whenthe temperature falls below the desired setting, a switch is closed toactivate the heating or cooling apparatus, which condition determinesthe inception of an air turbulence condition. Shown coupled tothermostat 40 is a switch 41. Switch 41 is normally included within thethermostat 14. In any event, when the heating system is not operating,switch 41 is open as shown, as soon as the thermostat indicates a needfor operation of the heating system, switch 41 is closed, as shown inthe dashed line position.

It is, of course, understood that this description is by way of exampleonly and the switch can operate in any other manner to achieve thedesired results. In any event, closure of switch 41 places a groundpotential on the input of a gate as a transistor or diode logic circuit43, while simultaneously energizing a source of infrared energy,depicted as a lamp assembly 44. Such infrared sources as 44 arenumerous, as light emitting diodes, lamps and so on and many examples oflight beam apparatus are well known and available. While FIG. 2 showsone such source 44, it is understood that switch 41 can activate aplurality of such sources.

The radiation emanating from source 44 is directed about the area to beprotected by suitable lens systems and so on and caused to impinge upona number of strategically placed photo detectors as 45, 46 and 47. Eachdetector as 45, 46 and 47 is illuminated by a source as 44; if anintruder caused the interruption of the radiation by placing his personbetween the beam and the detector, an alarm indication is detectedaccording to a predesired interruption via detecting modules 48, 49 and50. The outputs of these detectors are coupled to a common OR gate 53Thus, if any one or more beams is interrupted, this indication istransmitted via gate 53 and thence via gate 54 as an alarm. It

is noted that gate 54 is enabled only when switch 41 is closed. Thus,during ultrasonic operation one cannot receive or generate an alarm fromthe back-up light beam system.

When switch 41 is closed, lead 60 disables the ultrasonic transmitter61. As indicated, there are numerous techniques for accomplishing this.

It is also seen that lead 60 is also directed to a NOR- MAL CONDITIONSIGNAL module 63. This module may be a conventional transistoroscillator and provides the exact normal signal to the communicationsline via OR gate 64, during back-up intrusion operation. Thus, closureof switch 41 turns on module 63 to thereby maintain a valid no intrusionsignal to the communications line or to prevent an alarm unit fromsounding. As soon as the heating unit is turned off via thermostat 40,switch 41 opens as shown. The back-up light system is deactivated byturning off the infrared source 44 and blocking gate 54 :as thiscondition appears as a beam interruption. Since gate 54 is deactivatedand also oscillator 63, there can be no spurious alarm signal. In thismode (switch 41 opened), the ultrasonic system is operating. Thustransmitter 61 is propogating ultrasonic signals which are monitored forDoppler shifts by receiver and detector module 70. If an intrusionexists, an alarm signal is provided conventionally, transmitted tomodule 71 and thence via gate 64 to central control or to an alarm unit.

Thus, FIG. 2 depicts the above described modes, namely, when turbulenceis occuring the ultrasonic system is inactive and hence one does notfear false Doppler signals. In any event, a back-up system isimplemented and operating. In regard to central control, all

conditions appear normal, exee r'ir an intrudei' is within the area tobe protected and he disrupts a light beam, an alarm is sent. When theheating system is off, the back-up system is inactivated and theultrasonic system operates to provide protection. It is understood thatunder most normal conditions, this will be the case and ultrasonicprotection will afford the majority of operating time protection.

In any event, it is also clear that due to the random operation of thethermostatically controlled heating system as further controlling boththe ultrasonic system ndin 2% .r923m maasaebiexzseasstaiwte q beyond thecomplete discrimination of air turbulence.

Clearly one of the prime disadvantages of a light beam security systemis that an intruder can monitor the premises with the use of specialglasses and accurately determine, prior to entry, where the beams arepositioned. In the above system this is extremely difficult as the beamsare activated only during the time the ultrasonic system is off.

Furthermore, the beams are off when the ultrasonic system is on, andeven if an intruder was successful in locating and thence avoiding thebeams, he would eventually be detected during the ultrasonic mode.

FIG. 3 is included to show the diversity of the system and to depict analternate mode of implementing operation, it being understood thatnumerous alternatives are so available to one skilled in the art.

Briefly, a thermostat 80 monitors the environment v and as desired,operates the heating or cooling system.

When such operation is not afforded, switch 81 closes inactivating theultrasonic system via gate or amplifier 82. A plurality of infrared orother light sources 83, 84 and 85 are energized with switch 81 closure.Thus, a plurality of beams of radiation are propagated to photodetectors86 strategically positioned about the area. If an intruder interrupts abeam, the photodetector 86 detects interruption and via a detector as 87or 88 energizes a common gate 89. Gate 89 then activates module 90designated as a Doppler modulator. The function of 90 being to providethe normal signal immediately upon switch 81 closure via activation ofthe modulator 90 by gate 91. Thus the modulator 90 with an oscillator95, prior to activation of gate 89 provides the normal signal to thealarm via oscillator 95 as coupled to an alarm gate 100. As soon as gate89 indicates a beam interruption modulator 90 causes oscillator 95 to bemodulated with a Doppler shift, thus gate 100 provides the exact signalto the communications control as would be provided via lead during anintruder detection signal as provided by the ultrasonic system.

It is, of course understood that the thermostat control can be alternateto the normal control and the user can have the option of keeping thetemperature at a mini- 'a'cc'ordingto environmental conditions of saidarea,"

and which air turbulence can undesireably cause said intrusion system tofalsely indicate the presence of said intruder, comprising:

a. first means including a thermostat responsive to the operation ofsaid selectively operated generating source to provide a control signalindicative of said operation and therefore of said air turbulence,

b. second means coupled to said intrusion system and responsive to saidcontrol signal for deactivating said system during the presence of saidcontrol signal, whereby said intrusion system is monitoring said areaonly when said selectively operated source is not operating asdetermined by said environmental condition.

2. The apparatus accordng to claim 1 wherein said intrusion detectionsystem is of the ultrasonic type.

3. The apparatus according to claim 1 wherein said generating source ofair current is a forced air heating or cooling means.

4. An ultrasonic intrusion detection system of the type employing anultrasonic transmitter for propagating an ultrasonic signal within anarea to be protected and a receiver responsive to a change in saidtransmitted signal evidenced by a Doppler shift caused by the movementof an intruder within said area, in combination therewith apparatus forselectively energizing said system during the absence of propagating aircurrents which undesirably produce false Doppler shifts, comprising:

a. a thermostat to provide a control signal indicative of the presenceof propagating air currents, and

b. selectively operated switching means coupled to said ultrasonicsystem and operative in a first mode to deactivate said system inresponse to the generation of said control signal and in a second modeto activate said system during the absence of said air currents asmonitored by said monitoring means.

5. The apparatus according to claim 4 further comprising:

a. a back-up intrusion system capable of being selectively activated anddeactivated and operative when activated to protect said area by meansuneffected by said air currents, and

b. means coupling said back-up intrusion system to said selectivelyoperated switching means to activate said system only during said firstmode, to thereby assure said area is continuously protected.

6. The apparatus according to claim 4 where said propagating aircurrents are generated by a forced air heating, air conditioning unitemploying a fan causing 65 said propagating air currents to circulate.

7. The apparatus according to claim 6 wherein said monitoring meanscomprises a thermostat responsive to the temperature within said area asdetermined by the operation of said forced air heating or airconditioning unit.

8. An intrusion detection system, comprising:

a. an ultrasonic intrusion detection system employing b. a source ofcirculating air currents operative in a first mode to alter thetemperature of said area by circulating forced air currents and in asecond stand-by mode determined by the absence of said air currents,said air currents characterized by possessing a velocity indicative ofDoppler shifts,

. monitoring means including a thermostat responsive to the operation ofsaid source in said first mode to provide a control signal indicative ofsuch operation,

. control means responsive to said control signal and coupled to saidintrusion system to selectively deactivate said intrusion system onlyduring the presence of said control signal whereby said intrusion systemis operative only during the absence of said interfering air currents.

9. The system according to claim 8 further compris- 10 ing:

a. a communications channel coupled to said intrusion system and havingan input end responsive to the generator of said Doppler shift toprovide a predetermined signal at an output end of said channel forindicting the presence of said intruder,

b. means coupling said intrusion detection system to said input end ofsaid channel, said means including means for providing a normaloperating signal at said input end when said Doppler shift is notgenerated,

c. a selectively operated back-uip intrusion system of the typeunaffected by the propagation of said air currents and responsive tosaid control signal to be selectively operated during said first modeand to be deactivated during the absence of said control signal, and

d. means coupling said back-up intrusion system to said input end ofsaid communications channel during said first mode.

10. The apparatus according to claim 9 wherein said back-up system is ofthe radiation beam generating type for providing a beam of energy aboutsaid area, the interruption of said beam being indicative of thepresence of an intruder.

1. In an intrusion detection system for monitoring a predetermined area,said system, when activated is of the type employing a selectedfrequency transmitter and a receiver responsive to said transmittedfrequency as determined by a Doppler shift due to the presence of anintruder in said area, the combination therewith including apparatus foroperating said system in said area in the presence of a generatingsource of air current turbulence, which source is selectively operatedaccording to environmental conditions of said area, and which airturbulence can undesireably cause said intrusion system to falselyindicate the presence of said intruder, comprising: a. first meansincluding a thermostat responsive to the operation of said selectivelyoperated generating source to provide a control signal indicative ofsaid operation and therefore of said air turbulence, b. second meanscoupled to said intrusion system and responsive to said control signalfor deactivating said system during the presence of said control signal,whereby said intrusion system is monitoring said area only when saidselectively operated source is not operating as determined by saidenvironmental condition.
 2. The apparatus accordng to claim 1 whereinsaid intrusion detection system is of the ultrasonic type.
 3. Theapparatus according to claim 1 wherein said generating source of aircurrent is a forced air heating or cooling means.
 4. An ultrasonicintrusion detection system of the type employing an ultrasonictransmitter for propagating an ultrasonic signal within an area to beprotected and a receiver responsive to a change in said transmittedsignal evidenced by a Doppler shift caused by the movement of anintruder within said area, in combination therewith apparatus forselectively energizing said system during the absence of propagating aircurrents which undesirably produce false Doppler shifts, comprising: a.a thermostat to provide a control signal indicative of the presence ofpropagating air currents, and b. selectively operated switching meanscoupled to said ultrasonic system and operative in a first mode todeactivate said system in response to the generation of said controlsignal and in a second mode to activate said system during the absenceof said air currents as monitored by said monitoring means.
 5. Theapparatus according to claim 4 further comprising: a. a back-upintrusion system capable of being selectively activated and deactivatedand operative when activated to protect said area by means uneffected bysaid air currents, and b. means coupling said back-up intrusion systemto said selectively operated switching means to activate said systemonly during said first mode, to thereby assure said area is continuouslyprotected.
 6. The apparatus according to clAim 4 where said propagatingair currents are generated by a forced air heating, air conditioningunit employing a fan causing said propagating air currents to circulate.7. The apparatus according to claim 6 wherein said monitoring meanscomprises a thermostat responsive to the temperature within said area asdetermined by the operation of said forced air heating or airconditioning unit.
 8. An intrusion detection system, comprising: a. anultrasonic intrusion detection system employing a selectively operatedtransmitter, receiver apparatus for respectively transmitting anultrasonic frequency and receiving and detecting a Doppler shiftindicative of the presence of an intruder in a predetermined area to beprotected, b. a source of circulating air currents operative in a firstmode to alter the temperature of said area by circulating forced aircurrents and in a second stand-by mode determined by the absence of saidair currents, said air currents characterized by possessing a velocityindicative of Doppler shifts, c. monitoring means including a thermostatresponsive to the operation of said source in said first mode to providea control signal indicative of such operation, d. control meansresponsive to said control signal and coupled to said intrusion systemto selectively deactivate said intrusion system only during the presenceof said control signal whereby said intrusion system is operative onlyduring the absence of said interfering air currents.
 9. The systemaccording to claim 8 further comprising: a. a communications channelcoupled to said intrusion system and having an input end responsive tothe generator of said Doppler shift to provide a predetermined signal atan output end of said channel for indicting the presence of saidintruder, b. means coupling said intrusion detection system to saidinput end of said channel, said means including means for providing anormal operating signal at said input end when said Doppler shift is notgenerated, c. a selectively operated back-up intrusion system of thetype unaffected by the propagation of said air currents and responsiveto said control signal to be selectively operated during said first modeand to be deactivated during the absence of said control signal, and d.means coupling said back-up intrusion system to said input end of saidcommunications channel during said first mode.
 10. The apparatusaccording to claim 9 wherein said back-up system is of the radiationbeam generating type for providing a beam of energy about said area, theinterruption of said beam being indicative of the presence of anintruder.